NFS FAQ (http://nfs.sourceforge.net):
D11. Why doesn't "mount -oremount,tcp" convert an NFS-mounted file system
mounted with UDP to one mounted with TCP?
A. The "remount" option on the mount command only affects the generic mount
options, such as ro/rw, sync, and so on (see man mount for a complete list of
generic mount command options). The NFS-specific mount options listed on the nfs
man page can't be changed with a "mount -oremount" style mount command. You must
unmount your file system and mount it again with new options in order to modify
the NFS-specific settings.
Note that the mount command may update the contents of /etc/mtab whether or
not the actual mount settings have changed in the kernel. So when you try mount
-oremount with an NFS-specific mount option, subsequent mount commands may
report that the setting is in effect. This is only because the mount command is
reading /etc/mtab. The /proc/mounts file reflects the true mount options that
the kernel is using.

Perhaps I should have listed this as an RFE, not a bug. I can't think of any
reason why certain NFS options (such as the retrans limit) couldn't be changed
on the fly without requiring a umount (though others, such as udp->tcp very well
might require a umount first). Would it be better for me to file the RFE upstream?